Time-of-flight information on PET systems (TOFPET) allows for unprecedented sensitivity, as the signal-to-noise ratio and thus the background rejection are very much improved. The very high gain of the silicon multiplier (SiPM) and its sensitivity to single photon hits makes it a good candidate for highly compact systems. A timing resolution of 200 ps is enough to confine the event origin with a FWHM position uncertainty of 30 mm along the line-of-response (LOR). Achieving this fine resolution calls for fast front-end electronics, capable of extracting a very precise time stamp of each event. Scintillation light statistics, which include intrinsic timing characteristics of the crystal and the travel path of the photons, along with the time drift inherent to the e-h pair generation of the SIPM, may become a source of jitter that could ultimately compromise the targeted time resolution. In fact, the signal shape fluctuation at the output of the photodetector reflects the statistical time distribution of each photon building up the signal. Since the arrival time of these photons is weakly correlated to the time of the electron-positron annihilation, the readout system must be able to trigger on the first photo-electron. This ability requires low-noise front-end electronics with enough bandwidth such that the time walk across the dynamic range becomes negligible.
On the other hand, the design of compact PET detectors poses strict limits on power consumption. This constraint has motivated the choice of a low-power input stage and a very low-power analogue time-to-digital converter with a time binning of 50 ps.